The present invention relates to diagnostic systems, and more particularly to a diagnostic system for a digital signal processor which monitors various internal test points of the processor to determine that small portion of the total circuitry which is the source of an error.
When a signal processor fails, it is the job of a maintenance or service person to restore the processor to working order as quickly as possible. The first task in such a job is to generally find the area of the processor within which the malfunction occurred, i.e., the circuit board or module level, and then to replace that module with a spare. This allows the processor to be put back into service while the problem module is repaired off-line or during a planned maintenance period. Even where there are no spare modules available so that repairs have to be made on-line, the ability to quickly narrow the fault to a limited range of circuitry greatly speeds the repair process, decreasing the overall down-time of the processor.
Most problems in signal processors involve a known good signal, or set of signals, at the input of the processor generating a bad signal, or set of signals, at the output. By tracing along the internal signal path of the processor from input to output the service person can usually determine the faulty circuit by noting when the good signal turns bad. Conversely the signal path may be traced from the output toward the input and noting when the bad signal becomes good. If this tracing is done in discrete steps, for example at the input and output of each functional or physical module, the faulty module can quickly be identified as the circuitry between here the signal was good and where it started to go bad.
When the processor processes signals in analog form, the signal path is relatively easy to follow since the signal usually is represented by a single wire or point. This point can be probed with various types of test equipment, such as oscilloscopes and the like, to give the service person an easy means for reading the status of the signal. When the internal processing is digital, however, a given signal may exist as many discrete wires, or many bits of data multiplexed on a single wire, all of which must be simultaneously taken into account to derive an indication of that signal's status. Except for the most trivial problems, this is beyond the ability of oscilloscopes and usually requires more complex test equipment, such as logic analyzers and the like.
These more complex test instruments take longer to set up, since individual probes must be attached to each wire of the signal path, and are usually limited to single "snapshots" of a signal at a time. The "snapshot" is then analyzed by the service person. If the problem is intermittent in nature, the chance of having the failure occurring during one of the "snapshots" is low, adding to the total service time.
A desirable alternative world be to display various test points in a form with which the service person is familiar. In the case of a digital audio signal processor this might be analog audio. In the case of a digital video signal processor the preferred output would be video pictures. Since the original fault was probably initially identified by listening to or looking at the output of the processor, if the internal test points could also be displayed at the output on the same monitoring equipment, it would aid in recognizing whether the signal at a given test point is good or bad.